CCFLs powered by DC-AC inverters have been dominant in medical patient monitors, but LED BLUs are on the horizon and gaining ground in terms of usage. The benefits of each differ across many areas, including functionality, reliability, and cost. This article showcases each technology and provides the applications in which they are each best suited.
Tom Novitsky is president and Bill Abbott is director of sales and marketing at Endicott Research Group. They can be reached at 800-215-5866 or firstname.lastname@example.org.
Medical diagnostic devices have become powerful state-of-the-art instruments for instantaneous display of vital patient information. Key to their effectiveness are monitors to display the critical information they provide. Devices such as defibrillators, cardiac monitors, vital signs monitors, and full-clinical-parameter bedside and portable monitors demand that the information displayed be clear, crisp, and easily readable.
Medical devices present a special set of requirements for the LCD BLU and the backlight inverter. The LCD needs to provide high contrast, high resolution images, and data, as well as a resistance to glare. Portable medical devices must provide a bright, long lasting image when powered by batteries. Displays incorporated into devices used in emergency vehicles must withstand extreme temperatures and conditions, including shock, vibration, and even harsh environments, such as being sprayed with a fire hose.
Understanding the Display Backlight
Depending on the application, the DC to AC inverter that powers the CCFL backlight can have an input voltage ranging from 5 to 48 VDC. The inverter provides the high AC voltages required to light the CCFL and maintains the constant AC current required to control the CCFL brightness.
Controlling Image QualityThe quality of the displayed image depends heavily on the inverter. The LCD manufacturer or distributor supplies the LCD panel to the medical device manufacturer with a CCFL BLU that meets the application requirements. The medical device manufacturer must choose an inverter that will provide acceptable brightness, lamp life, dimming features, and dependable performance. The device manufacturer may contact the LCD manufacturer for an inverter recommendation or may talk to a field application engineer (FAE) at the LCD distributor. Another, and perhaps better option is to contact an engineer at the inverter manufacturer. Inverter manufacturers are familiar with all of the panels from the major LCD manufacturers and can offer expertise in recommending the optimum inverter.
LEDThe New Monitor Backlighting Technology
LEDs are already used to backlight a wide range of smaller displays (e.g., cell phones and MP3 players). Because of their higher power consumption and, in some cases, their mercury content, CCFL BLUs are beginning to be replaced by LED BLUs for mid-size and even larger displays.
An LED BLU is generally driven by a constant current and does not require an alternating current at high voltage, therefore an inverter is not required. This does not mean that LED BLUs do not require appropriately designed drivers to maintain constant brightness and to provide dimming. An LED's efficiency depends on the amount of current flowing through it and, therefore, for quality performance in a BLU application, a constant current driver is required to maintain constant brightness. To compensate for changes in the voltage of an LED string, which may occur with changing temperature, or may vary with tolerances across a large number of LED backlights, drivers must be designed with the ability to adapt to changes in LED voltage.
LED AdvantagesLEDs can provide higher brightness than CCFLs. Lower efficiency (lumens/watt) LEDs may provide 20% more brightness than CCFLs. A typical increase in brightness might be 420 cd/m2 or "nits" for an LED BLU compared to 350 cd/m2 for the CCFL BLU version of the same LCD utilizing the same amount of power. High brightness, higher efficiency LEDs may provide 200% more brightness than conventional CCFL BLUs.
Also, LED backlights are typically more reliable and more durable over the lifetime of the display. LEDs are less fragile than CCFLs, and won't degrade if operated for long periods of time at cold temperatures (down to -30°C).
LED RailsEfficient thermal management (i.e., keeping the LEDs cool) is one of the major design challenges for LED BLUs. The LED BLU rail shown in Figure 1 and Figure 4 incorporates the LEDs on a long, narrow PC board that is thermally bonded to a metal channel or "rail" that is similar to the channel used to house the CCFLs. The thermal management technology utilized in the rail assembly addresses the challenge of efficient thermal management by keeping the LEDs cool. The design provides a technologically more efficient way to conduct heat away from the LEDs to maintain a low LED junction temperature, which is critical to long-life operation of the LED BLU.
Driving the LED RailsA wide variety of LED driver ICs are becoming available for use in LED driver circuits. Most of these driver ICs employ switching and/or linear circuits to control the LED current. Depending on the configuration and number of strings, there is generally an optimum topology choice for a given display backlight. Many times, the voltage available is not usable to drive the wide array of string voltages that are required from display to display; therefore, it is not possible to use the standard +5 or +12 V available with a driver IC chip to drive the backlight. Many LED drivers employ boost or buck circuits to either increase or decrease the supply voltage, and they also employ a secondary circuit that will directly drive the LEDs to a constant current. High power backlights are often driven by a different topology than a lower power backlight, as one topology may be more optimized than another for a given power level. Along with making the correct topology choice, those developing a backlight driver need to be well versed in power supply layout for switching and linear circuits. The designer will also face EMI and thermal issues if the proper components are not chosen and the layout is not optimized.
LED drivers are now available as standard products for a broad range of LCDs, including the 6.4? to 12.1? sizes commonly used in medical device displays. (See Figure 4.) These drivers provide full function in a compact size, with wide range dimming, wide input voltages, and full brightness and enable controls. Available in a range of sizes, input voltages, and dimming ratios, they are compatible with most OEM LED-backlit panels or can also be used with the Smart Force LED rails shown in Figure 1 and Figure 4.
Cost of LED versus CCFL
One option is to use an LED-backlit LCD from an LCD manufacturer. These LCDs provide slightly higher brightness than the CCFL equivalents or lower input power at the same brightness as the CCFL equivalents. The LED drivers previously described will provide the control needed to optimize display operation.
Another option is to retrofit the CCFL BLU to an LED BLU. This can provide a solution for LCDs already in the field. The LED BLU rail shown in Figure 1 is designed as a drop-in replacement for CCFL BLU rails. When used with compatible LED drivers, the LED rails provide the often simpler CCFL to LED retrofit option.
A major manufacturer of patient monitoring solutions was looking for drop-in replacement inverters for their high-performance transportable patient monitors, which are backlit with a CCFL BLU. When presented with the option of an LED rail/driver retrofit solution that could be installed in the field by their service personnel, they decided to consider the performance advantages of LED backlights now instead of in the future, regardless of the higher cost of the LED BLU.
LED BLUs using Smart Force LED rails with lower-efficiency LEDs can cost twice as much as comparable CCFL BLUs, while those using higher efficiency, higher-brightness LEDs (H/B LEDs) can cost five times the price of a CCFL BLU. However, there is also a proportional increase in performance. In the former case, the LED BLU will provide the same brightness with 20% less power, or 20% more brightness at the same power. In the latter case, LED BLUs using H/B LEDs can provide twice the brightness with as much as 50% less power The high efficiency of the drivers combined with rails with built-in thermal management to keep the LEDs running cool provide ~50,000 hour LED BLU lifetimes.
Costs for the LED drivers for the lower efficiency LED BLUs are comparable to costs for CCFL inverters. An LED driver for an OEM LED-backlit panel that has previously used a $15.00 DC-AC inverter would be approximately the same. Costs for LED drivers using the higher efficiency LED BLUs are higher but also provide much higher brightness.